Aircraft environmental control system

ABSTRACT

An aircraft environmental control system (4) for controlling a temperature of a fluid, the control system (4) comprising: a temperature sensor (30) configured to measure a temperature of the fluid and to generate a first signal, the first signal being indicative of the measured temperature; a control signal generator (52) configured to, dependent upon the first signal, generate a control signal for controlling the temperature of the fluid; and one or more processors (64) configured to, responsive to determining that the measured temperature is less than or equal to a pre-determined threshold value (e.g. 0° C.), increase a gain of the control signal generator (52).

RELATED APPLICATIONS

This application is a national phase application filed under 35 USC §371 of PCT Application No. PCT/GB2016/053412 with an Internationalfiling date of Nov. 3, 2016 which claims priority of GB PatentApplication 1519607.4 filed Nov. 6, 2015 and EP Patent Application15275229.1 filed Nov. 6, 2015. Each of these applications is hereinincorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to aircraft environmental control systems.

BACKGROUND

Many aircraft include environmental control systems.

Environmental control systems may be used to provide thermal control toother aircraft subsystems including, but not limited to, an aircrewworkspace and equipment on-board the aircraft. For example,environmental control systems may be used to provide cooling air to acockpit and/or mission computers of an aircraft.

Environmental control systems may provide thermal control of otheraircraft subsystems by mixing relatively hot air that is bled from, forexample, a compressor stage of a gas turbine engine of the aircraft(i.e. “bleed air”) with relatively cold, refrigerated air.

Many aircraft, for example Hawk aircraft, include analogue environmentalcontrol systems.

SUMMARY OF THE INVENTION

The present inventors have realised that temperatures on an aircrafttend to be dependent upon a throttle setting of the aircraft engine.

The present inventors have further realised that air flows experiencedby the aircraft tend to be dependent upon a throttle setting of theaircraft engine.

The present inventors have further realised that it would be beneficialto improve the responsiveness of the environmental control systems onmany aircraft (for example on Hawk aircraft) to facilitate accountingfor typical changes in throttle setting during aircraft operation.

In a first aspect, the present invention provides an aircraftenvironmental control system for controlling a temperature of a fluid(e.g. a conditioned air supply for supplying cooling/heating to systemson board the aircraft). The control system comprises: a temperaturesensor configured to measure a temperature of the fluid and to generatea first signal, the first signal being indicative of the measuredtemperature; a control signal generator configured to, dependent uponthe first signal, generate a control signal for controlling thetemperature of the fluid (e.g. to control means for controlling thetemperature of the fluid); and one or more processors configured to,responsive to determining that the measured temperature is less than orequal to a pre-determined threshold value, increase a gain of thecontrol signal generator.

The pre-determined threshold value may be less than or equal to 0° C.

The fluid may be a mixture comprising a second fluid and a third fluid.The environmental control system may further comprise a valve for mixingthe second fluid and the third fluid. The control signal generator maybe configured to, using the first signal, generate a control signal forcontrolling the valve. The one or more processors may be configured to,responsive to determining that the measured temperature is less than orequal to the pre-determined threshold value, increase a gain of thecontrol signal generator, thereby increasing an amplitude and/or powerof the control signal generated by the control signal generator. Thesecond fluid may comprise bleed air from a subsystem of the aircraft.The third fluid may comprises refrigerated bleed air and/or ambient air.

The one or more processors may be further configured to: determine thatthe measured temperature is oscillating about a central value; and,responsive to the determining that the measured second temperature isoscillating about a central value, decrease the gain of the controlsignal generator.

The aircraft environmental control system may further comprise: one ormore control laws, the control laws being for use by the one or moreprocessors when generating the control signal; a transmitter configuredto transmit, from the aircraft environmental control system, for use byone or more entities remote from the aircraft environmental controlsystem, performance data, the performance data including data selectedfrom the group of data consisting of: one or more measured values of thetemperature, data indicative of the first signal, data indicative of again of the control signal generator, and data indicative of the controlsignal; and a receiver configured to receive, responsive to thetransmitter transmitting the performance data, from the one or moreentities remote from the aircraft environmental control system, updateinformation for use by the one or more processors; and means for, usingthe update information, updating the one or more control laws.

The control system may further comprise an amplifier configured toamplify the first signal. The control signal generator may be configuredto generate the control signal using the amplified first signal. The oneor more processors may be further configured to, responsive todetermining that a temperature of an operational environment of theamplifier is not within a predetermined temperature range, modify a gainof the amplifier. The aircraft environmental control system may furthercomprise: a housing; and a second temperature sensor configured tomeasure the temperature of the operational environment of the amplifierwithin the housing. The amplifier, the second temperature sensor, andthe one or more processors may be located within the housing. Theaircraft environmental control system may further comprise a baselinesignal generation module configured to generate a baseline signal, thebaseline signal being independent of the temperature of the operationalenvironment of the amplifier. The amplifier may be further configured toamplify the baseline signal. The one or more processors may be furtherconfigured to, responsive to determining that the temperature of anoperational environment of the amplifier is not within the predeterminedtemperature range, modify the gain of the amplifier using the amplifiedbaseline signal. The baseline signal generation module may comprise aresistor having substantially constant resistance. The baseline signalmay be indicative of a resistance of the resistor. The environmentalcontrol system may further comprise a relay switchable between a firstmode and a second mode. In its first mode, the relay may connect thefirst temperature sensor to the amplifier and disconnect the baselinesignal generation module from the amplifier. In its second mode, therelay may connect the baseline signal generation module to the amplifierand disconnect the first temperature sensor from the amplifier. The oneor more processors may be further configured to, dependent on themeasurements by the second temperature sensor, control the switching ofthe relay. The one or more processors may be further configured to,responsive to determining that the temperature of an operationalenvironment of the amplifier is not within the predetermined temperaturerange, control the relay to switch from its first mode to its secondmode.

The environmental control system may be on an aircraft, e.g. a Hawkaircraft.

In a further aspect, the present invention provides an aircraftcomprising an aircraft environmental control system according to anypreceding aspect.

In a further aspect, the present invention provides an aircraftenvironmental control method for controlling a temperature of a fluid.The method comprises: measuring a temperature of the fluid; generating afirst signal, the first signal being indicative of the measuredtemperature; generating, by a control signal generator, dependent uponthe first signal, a control signal for controlling the temperature ofthe fluid; determining that the measured temperature is less than orequal to a pre-determined threshold value; and responsive to determiningthat the measured temperature is less than or equal to thepre-determined threshold value, increasing a gain of the control signalgenerator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration (not to scale) of an aircraft;

FIG. 2 is a schematic illustration (not to scale) showing furtherdetails of an environmental control system of the aircraft;

FIG. 3 is a schematic illustration (not to scale) showing furtherdetails of a control module of the aircraft;

FIG. 4 is a process flow chart showing certain steps of a controlprocess of the environmental control system;

FIG. 5 is a process flow chart showing certain steps of an automaticcalibration process for the control module;

FIG. 6 is a process flow chart showing certain steps of an icingavoidance process;

FIG. 7 is a process flow chart showing certain steps of an oscillationavoidance process; and

FIG. 8 is a process flow chart showing certain steps of a process ofimproving control laws with which the environmental control system iscontrolled.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration (not to scale) of an aircraft 2 inwhich an embodiment of an environmental control system 4 is implemented.

In this embodiment, the aircraft 2 is a Hawk aircraft, for example a Mk127, Mk 128, Mk 132, Mk 165, Mk166, or later variant of a Hawk aircraft.

The aircraft 2 comprises the environmental control system 4 and one ormore aircraft subsystems that are to be cooled by the environmentalcontrol system 4. The one or more aircraft subsystems that are to becooled are hereinafter collectively referred to as the “aircraftsubsystems” and are indicated in FIG. 1 by a single box and thereference numeral 6.

The environmental control system 4 is described in more detail laterbelow with reference to FIG. 2.

The environmental control system 4 is connected to the aircraftsubsystems 6 via a bleed air channel 8. Bleed air from the aircraftsubsystems 6 is sent from the aircraft subsystems 6 to the environmentalcontrol system 4 via the bleed air channel 8. In particular, in thisembodiment the aircraft subsystems 6 includes an engine of the aircraft2. Bleed air from the aircraft engine, e.g. from a compressor stage of agas turbine engine of the aircraft 2, is sent from the engine to the tothe environmental control system 4 via the bleed air channel 8. Thebleed air may have a temperature of e.g. 350° C.

The environmental control system 4 is further coupled to an ambient airchannel 9. The ambient air channel 9 is in fluid communication with airexternal to the aircraft 2 such that the environmental control system 4receives ambient air via the ambient air channel 9. The ambient airchannel 9 may include a ram air inlet via which ambient air enters theambient air channel 9.

In this embodiment, as described in more detail later below, the ambientair is used to cool a portion of the bleed air to produce “cooling air”.This relatively cold cooling air is combined with another portion of therelatively hot bleed air to produced “conditioned air”.

The environmental control system 4 is further connected to the aircraftsubsystems 6 via a conditioned air channel 10. The conditioned air issent from the environmental control system 4 to the aircraft subsystems6 via the conditioned air channel 10.

The aircraft subsystems 6 may include a cockpit of the aircraft 2.Conditioned air may be directed from the environmental control system 4to the aircraft cockpit via the conditioned air channel 10.

In this embodiment, the bleed air is air that has been heated by one ormore of the aircraft subsystems 6. The bleed air is relatively hotcompared to the cooling air and the conditioned air. The conditioned airsent from the environmental control system 4 to the aircraft subsystems6 is used to cool one or more of the aircraft subsystems 6.

FIG. 2 is a schematic illustration (not to scale) showing furtherdetails of the environmental control system 4.

In this embodiment, the environmental control system 4 comprises acontrol module 20, an air conditioning module 21, a valve 22, and atemperature measurement module 24.

The control module 20 is described in more detail later below withreference to FIG. 3.

The control module 20 is connected to the valve 22 via a firstelectrical connection 26 such that a valve control signal may be sentfrom the control module 20 to the valve 22. Determination of the valvecontrol signal by the control module 20 is described in more detaillater below with reference to FIGS. 3 and 4.

In this embodiment, the valve control signal is a pulse-width modulation(PWM), or pulse-duration modulation (PDM) signal.

In this embodiment, the valve control signal is a digital signal. Use ofa digital valve control signal tends to provide for improved valvecontrol compared to conventional analogue signals. The control module 20is further connected to the temperature measurement module 24 via asecond electrical connection 28. The control module 20 is connected tothe temperature measurement module 24 such that an electrical signal maybe sent from the temperature measurement module 24 to the control module20, as described in more detail later below.

The air conditioning module 21 is connected to the bleed air channelsuch that air conditioning module 21 receives some of the hot bleed airfrom the aircraft engine. The air conditioning module 21 is connected tothe ambient air channel 9 such that the air conditioning module 21receives the ambient air. The air conditioning module 21 cools thereceived hot bleed air by compressing, cooling (using the receivedambient air), and subsequently expanding the bleed air, therebyproducing cooling air. The air conditioning module 21 is furtherconnected to the valve 22 such that the air conditioning module 21 maysend the cooling air to the valve 22. In this embodiment, the coolingair has sub-zero temperature e.g. −30° C.

In this embodiment, the valve 22 is connected to the bleed air channel 8such that the valve 22 receives bleed air from the aircraft subsystems6. Also, the valve 22 is connected to the air conditioning module 21such that the valve 22 receives the cooling air. The valve 22 isconfigured to mix the received bleed air with the cooling air to producethe conditioned air. The valve 22 is operable such that the proportionsin which the bleed air and the cooling air are mixed may be varied.

The valve 22 is connected to the conditioned air channel 10. The valve22 outputs mixed bleed air and cooled conditioned air into theconditioned air channel 10 as the conditioned air. In other words, inthis embodiment the conditioned air is a mixture of the bleed air andthe cooling air.

In this embodiment, the valve 22 is controlled by the valve controlsignal sent to the valve 22 from the control module 20. The valvecontrol signal controls the position of the valve 22. In other words,valve control signal control controls the proportions in which the bleedair and the cooling air are mixed to produce the conditioned air.

In this embodiment, the temperature measurement module 24 is positionedwithin the conditioned air channel 10 downstream of the valve 22.

The temperature measurement module 24 comprises a thermistor 30.

The thermistor 30 is configured to measure a temperature of theconditioned air within the conditioned air channel 10 that has beenoutput by the valve 22. The temperature measurement module 24 isconnected to the control module 20 such that the temperature measurementtaken by the thermistor 30 is sent to the control module 20 via thesecond electrical connection 28.

In operation, an electrical signal indicative of the resistance of thethermistor 30 (i.e. that is indicative of the temperature of theconditioned air) is output by the temperature measurement module 24 tothe control module 20 via the second electrical connection 28.

FIG. 3 is a schematic illustration (not to scale) showing furtherdetails of the control module 20.

In this embodiment, the control module 20 comprises a housing 40, acontroller 42, a first electromagnetic compatibility (EMC) filter 44, asecond EMC filter 46, a third EMC filter 48, an isolating relay 50, avalve driver 52, a signal conditioning module 54, a memory 56, anin-service programming module 58, a transceiver 60, a power supplymodule 62, a processor 64, a clock oscillator 66, an internaltemperature sensor 68, a single pole, double throw (SPDT) relay 69, anda reference resistor 70.

The housing 40 contains the other components 42-70 of the control module20. In this embodiment, the housing 40 is made of metal, for example,aluminium. However, in other embodiments, the housing is made of adifferent appropriate material such as plastics.

In this embodiment, the controller 42 is an interface module that, inoperation, receives various signals from entities remote from thecontrol module 20. The controller 42 is configured to distribute thesignals received from these remote entities to relevant components ofthe control module 20. Also, in operation, the controller 42 receivesvarious signals from various other components of the control module 20.The controller 42 is configured to distribute the signals received fromother components of the control module 20 to relevant entities remotefrom the control module 20.

The controller 42 is connected to the first electrical connection 26such that the valve control signal may be sent from the controller 42 tothe valve 22 via the first electrical connection 26.

The controller 42 is coupled to the first EMC filter 44 such thatelectrical signals (in particular, the valve control signal in thisembodiment) may be sent between the controller 42 and the first EMCfilter 44.

The controller 42 is connected to the second electrical connection 28such that electrical signals may be sent between the controller 42 andtemperature measurement module 24 via the second electrical connection28.

The controller 42 is coupled to a first terminal of the SPDT relay 69such that electrical signals (in particular, the temperaturemeasurements in this embodiment taken by the thermistor 30) may be sentfrom the controller 42 to the first terminal of the SPDT relay 69.

The controller 42 is further configured to receive, via an inputconnection 71, a configuration signal. The configuration signal may bereceived by the controller 42 from a computer (such as a laptop, ortablet computer) remote from the aircraft 2, as described in more detaillater below with reference to FIG. 8.

The controller 42 is further configured to send, via an outputconnection 72, performance data. The performance data may be sent fromthe controller 42 to the computer that is remote from the aircraft 2, asdescribed in more detail later below with reference to FIG. 8.

The controller 42 is coupled to the isolating relay 50 such thatelectrical signals (in particular, the configuration signal and theperformance data in this embodiment) may be sent between the controller42 and the isolating relay.

The controller 42 is further configured to receive, via a power input74, electrical power. The electrical power may be 28V electrical power.The electrical power may be received by the controller 42 from a powersource remote from the control module 20 and on the aircraft 2.

In addition to being connected to the controller 42, the first EMCfilter 44 is connected to the valve driver 52 such that electricalsignals (in particular, the valve control signal in this embodiment) maybe sent between the first EMC filter 44 and the valve driver 52. Thefirst EMC filter 44 is configured to filter electrical signals receivedfrom the valve driver 52, thereby improving electromagneticcompatibility between the control module 20 and the valve 22.

In addition to its first terminal being connected to the controller 42,a second terminal of the SPDT relay 69 is connected to the referenceresistor 70 such that an electrical signal (in particular, a “baselinesignal”) may be sent from the reference resistor 70 to the secondterminal of the SPDT relay 69. In addition to the first and secondterminals, the SPDT relay 69 comprises a third terminal. The thirdterminal of the SPDT relay 69 is a common terminal that may be switchedfrom being connected to the first terminal to being connected to thesecond terminal and vice versa. In this embodiment, the third (i.e. thecommon) terminal of the SPDT relay 69 is connected to the second EMCfilter 46 such that electrical signals (in particular, depending uponthe state of the SPDT relay 69, either the temperature measurements ofthe thermistor or the baseline signal) may be sent from the thirdterminal of the SPDT relay 69 to the second EMC filter 46.

The SPDT relay 69 is further connected to the processor 64 such that arelay control signal for controlling the SPDT relay 69 may be sent fromthe processor 64 to the SPDT relay 69. The relay control signal controlsthe state of the SPDT relay 69, i.e. which of the first or secondterminals the third (common) terminal is connected to.

In this embodiment, the reference resistor 70 is a 10K precisionstabilised resistor. In other embodiments, a different means forgenerating a stable baseline signal is implemented instead of or inaddition to the reference resistor 70.

In addition to being connected to the third terminal of the SPDT relay69, the second EMC filter 46 is connected to the signal conditioningmodule 54 such that electrical signals (in particular, depending uponthe state of the SPDT relay 69, either the temperature measurements ofthe thermistor or the baseline signal) may be sent from the second EMCfilter 46 to the signal conditioning module 54. The second EMC filter 46is configured to filter electrical signals sent to the signalconditioning module 54, thereby improving electromagnetic compatibilitybetween the control module 20 and the temperature measurement module 24.

In addition to being connected to the controller 42, the isolating relay50 is connected to the in-service programming module 58 and thetransceiver 60 such that electrical signals may be sent between theisolating relay 50 and each of the in-service programming module 58 andthe transceiver 60. The isolating relay 50 is configured to direct tothe configuration signal to the in-service programming module 58. Also,the isolating relay 50 is configured to receive the performance datafrom the transceiver and direct the performance data to the controller42. In this embodiment, the isolating relay 50 is configured to isolatethe in-service programming module 58 and the transceiver 60 fromincoming signals while the aircraft is in flight. In other words, theisolating relay 50 prevents or opposes signals being input to thein-service programming module 58 and output by the transceiver 60 duringaircraft flight. Thus, updating of control laws 78 implemented by theprocessor 64 (which will be described in more detail later below) whilethe aircraft 2 is in flight tends to be prevented.

In addition to being connected to the controller 42, the third EMCfilter 48 is connected to the power supply module 62 such thatelectrical power may be sent from the third EMC filter 48 to the powersupply module 62. The third EMC filter 48 is configured to filterelectrical signals passing between the controller 42 and the powersupply module 62, thereby improving electromagnetic compatibilitybetween the control module 20 and the power source. The power supplymodule 62 is configured to supply electrical power to the othercomponent of the control module 42-60, 64-70.

In addition to being connected to the first EMC filter 44, the valvedriver 52 is connected to the processor 64 such that electrical signals(for example, an output of the processor 64) may be sent between thevalve driver 52 and the processor 64. The valve driver 52 is configuredto, using the output of the processor 64, determine the valve controlsignal.

In addition to being connected to the second EMC filter 46, the signalconditioning module 54 is connected to the processor 64 such thatelectrical signals may be sent between the signal conditioning module 54and the processor 64. The signal conditioning module 54 comprises anamplifier 76. In this embodiment, the amplifier 76 is configured toamplify a temperature measurement signal received by the signalconditioning module 54 from the thermistor 30. The signal conditioningmodule 54 is configured to output the amplified temperature measurementsignal to the processor 64. The signal conditioning module 54 maycomprise one or more filters and/or analogue-to-digital converters.

In this embodiment, the memory 56 is a non-volatile random accessmemory. The memory 56 is connected to the processor 64 such that theprocessor may store data (for example, performance data) in the memory,and such that the processor 64 may retrieve data from the memory 56.

In addition to being connected to the isolating relay 50, the in-serviceprogramming module 58 is connected to the processor 64 such thatelectrical signals (in particular, the configuration signals in thisembodiment) may be sent between the in-service programming module 58 andthe processor 64. The in-service programming module 58 is configured toprocess received configuration signals, and update or reconfigure thecontrol laws 78 using the received configuration signal.

In addition to being connected to the isolating relay 50, thetransceiver 60 is connected to the processor 64 such that electricalsignals (in particular, the performance data in this embodiment) may besent between the processor 64 and the transceiver 64. The transceiver 64is configured to transmit received performance data to the computer thatis remote from the aircraft 2 via the isolating relay 50.

In this embodiment, the processor 64 comprises the set of control laws78. The processor 64 implements the control laws 78 to process incomingsignals. The control laws 78 and the functionality of the processor 64is described in more detail later below with reference to FIGS. 4 to 7.The processor 64 is further configured to generate a relay controlsignal, and output the relay control signal to control the SPDT relay 69as described in more detail later below.

The clock oscillator 66 is configured to generate a clock signal. Theclock oscillator 66 is connected to the processor 64 such that theprocessor 64 may receive the clock signal from the clock oscillator 66.The clock signal is used to drive a clock of the processor 64.

The internal temperature sensor 68 is configured to measure atemperature inside the housing 40, i.e. to measure a temperature of theenvironment inside the housing 40. Thus, in this embodiment, theinternal temperature sensor 68 measures a temperature of the operationalenvironment of, inter alia, the amplifier 76. The internal temperaturesensor 68 is connected to the processor 64 such that the processor 64may receive the temperature measurement taken by the internaltemperature sensor 68. In this embodiment, the internal temperaturesensor 68 periodically, e.g. once per second.

Apparatus, including the processor 64, for implementing the abovearrangement, and performing the method steps to be described laterbelow, may be provided by configuring or adapting any suitableapparatus, for example one or more computers or other processingapparatus or processors, and/or providing additional modules. Theapparatus may comprise a computer, a network of computers, or one ormore processors, for implementing instructions and using data, includinginstructions and data in the form of a computer program or plurality ofcomputer programs stored in or on a machine readable storage medium suchas computer memory, a computer disk, ROM, PROM etc., or any combinationof these or other storage media.

FIG. 4 is a process flow chart showing certain steps of a controlprocess for controlling a temperature of the conditioned air in theconditioned air channel 10.

At step s2, the valve 22 receives bleed air from the aircraft subsystems6 via the bleed air channel 8, and cooling air from the air conditioningmodule 21. The valve 22 further receives a valve control signal from thevalve driver 52 via the first EMC filter 44, the controller 42, and thefirst electrical connection 26.

The valve control signal specifies a rate of change of position of thevalve 22.

At step s4, the valve 22 operates in accordance with the received valvecontrol signal. The proportions in which the hot bleed air and thecooling air are mixed is dependent upon the position of the valve 22.The valve 22 mixes the bleed air and the cooling air in accordance withthe received valve control signal to produce the conditioned air.

At step s6, the valve outputs the conditioned air (i.e. mixture of thebleed air and the cooling air) to the conditioned air channel 10.

At step s8, the thermistor 30 located in the conditioned air channel 10measures a temperature of the conditioned air output by the valve 22.

At step s10, the temperature measurement module 24 sends the measurementof the temperature of the conditioned air to the processor 64 of thecontrol module 20.

In particular, in this embodiment, the temperature measurement module 24sends the measurement of the temperature of the conditioned air tocontroller 42 as an electrical signal via the second electricalconnection 28. This measurement signal is sent by the controller 42 tothe signal conditioning module 54 via the SPDT relay 69 (which is in astate whereby the first terminal is connected to the third terminal) andthe second EMC filter 46. The signal conditioning module 54 processesthe measurement signal to be in an appropriate format for the processor64. This includes amplifying the measurement signal by the amplifier 76.The signal conditioning module 54 then sends the processed measurementsignal to the processor 64.

The processor 64 may store the received measurements of the temperatureof the cooling air at the memory 56 as performance data.

At step s12, the conditioned air passes to the aircraft subsystems 6 viathe conditioned air channel 10, thereby cooling the aircraft subsystems6.

At step s14, using the received measurement of the temperature of theconditioned air, the processor 64 and the valve driver 52 determine anupdated valve control signal for controlling the position of the valve22.

In particular, in this embodiment the processor 64 uses the control laws78 to process the measurement of the temperature of the conditioned air.

In this embodiment, the control laws 78 include laws and/or rules thatspecify the following.

Firstly, the control laws 78 specify that, if the measured value of thetemperature of the conditioned air is within a first predeterminedtemperature range, the current position of the valve 22 is to bemaintained. In this embodiment, the first predetermined temperaturerange is the range 4° C.+/−2° C., i.e. between 2° C. and 6° C. However,in other embodiments, the first predetermined temperature range may be adifferent range of temperature values.

Secondly, the control laws 78 specify that, if the measured value of thetemperature of the conditioned air is below the first predeterminedtemperature range, the position of the valve 22 is to be changed toincrease the proportion of the relatively hot bleed air in theconditioned air, and to decrease the proportion of the cooling air inthe conditioned air. Thus, if the measured value of the temperature ofthe conditioned air is below the first predetermined temperature range,the valve 22 is to be controlled so that the temperature of theconditioned air is increased. In this embodiment, the change in positionof the valve 22 may be proportional to the measured conditioned airtemperature. For example the control laws 78 may specify that, if themeasured value of the temperature of the conditioned air is below thefirst predetermined temperature range by a relatively small amount, theposition of the valve 22 is to be changed by a relatively small amountso that the proportion of the bleed air in the conditioned air isincreased by a relatively small amount. The control laws 78 may furtherspecify that, if the measured value of the temperature of the coolingair is below the first predetermined temperature range by a relativelylarge amount, the position of the valve 22 is to be changed by arelatively large amount so that the proportion of the bleed air in theconditioned air is increased by a relatively large amount. The controllaws 78 may include a first function that specifies an amount by whichto change the position of the valve 22 if the measured value of thetemperature of the conditioned air is below the first predeterminedtemperature range. The first function may be function of the measuredvalue of the temperature of the conditioned air. In some embodiments,the first function is a linear function.

Thirdly, the control laws 78 specify that, if the measured value of thetemperature of the conditioned air is above the first predeterminedtemperature range, the position of the valve 22 is to be changed todecrease the proportion of the relatively hot bleed air in theconditioned air, and to increase the proportion of the cooling air inthe conditioned air. Thus, if the measured value of the temperature ofthe conditioned air is above the first predetermined temperature range,the valve 22 is to be controlled so that the temperature of theconditioned air is decreased. In this embodiment, the change in positionof the valve 22 may be proportional to the measured conditioned airtemperature. For example the control laws 78 may specify that, if themeasured value of the temperature of the conditioned air is above thefirst predetermined temperature range by a relatively small amount, theposition of the valve 22 is to be changed by a relatively small amountso that the proportion of the cooling air in the conditioned air isincreased by a relatively small amount. The control laws 78 may furtherspecify that, if the measured value of the temperature of theconditioned air is above the first predetermined temperature range by arelatively large amount, the position of the valve 22 is to be changedby a relatively large amount so that the proportion of the cooling airin the conditioned air is increased by a relatively large amount. Thecontrol laws 78 may include a second function that specifies an amountby which to change the position of the valve 22 if the measured value ofthe temperature of the conditioned air is above the first predeterminedtemperature range. The second function may be function of the measuredvalue of the temperature of the conditioned air. In some embodiments,the second function is a linear function.

Furthermore, in this embodiment, the control laws 78 specify that, ifone or more certain criteria are met, a brief maximum PWM signal isapplied to the valve 22. Examples of such criteria include, but are notlimited to, a criterion that the valve 22 is to change direction, or toa criterion that the valve 22 is to open from a static condition. Thisapplication of a maximum PWN signal as the valve control signal to thevalve 22 advantageously tends to overcome inertia of the valve 22.

Thus, in this embodiment the processor 64 implements the control laws 78and processes the measurement of the temperature of the cooling air todetermine a rate in which to change the position of the valve 22. Theprocessor 64 then sends the rate of change of position of the valve 22to the valve driver 52.

Using the received position rate of change of position of the valve 22,the valve driver 52 then generates an updated valve control signal. Theupdated valve control signal specifies the rate of change of position ofthe valve 22 as determined by the processor 64.

In this embodiment, the processor 64 determines a rate of change ofposition of the valve 22. Thus, advantageously positional sensing on thevalve 22 tends not to be used. However, in other embodiments, positionalsensing on the valve 22 is used. In such embodiments, the processor 64may determine an updated position for the valve 22, and the valvecontrol signal generated by the valve driver 52 may specify thisdetermined valve position.

At step s16, the valve driver 52 of the control module 20 sends theupdated valve control signal to the valve 22. The valve control signalis sent from the valve driver 52 to the valve 22 via the first EMCfilter 44, the controller 42, and the first electrical connection 26.

The processor 64 may log details of the generated valve control signalat the memory 56 as performance data.

After step s16, the method of FIG. 3 may return to step s2 at which thevalve 22 may be repositioned in accordance with the updated valvecontrol signal. Thus, the relative proportions of bleed air and coolingair that make up the conditioned air may be changed so that thetemperature of the conditioned air is maintained within the firstpredetermined temperature range. Steps s2 to s16 define a control loopfor controlling the valve 22 so that the temperature of the conditionedair is within the first predetermined temperature range.

Thus, a control process for controlling a temperature of the conditionedair in the conditioned air channel 10 is provided.

FIG. 5 is a process flow chart showing certain steps of an automaticcalibration process performed by the control module 20. The process ofFIG. 5 may be performed at regular intervals, e.g. at a frequency of 20Hz. This tends to ensure that the control module is correctly calibratedat all times.

Large changes in the temperature of the internal circuitry of thecontrol module 20 can detrimentally affect the ability of the controlmodule to correctly measure or determine the temperature of thethermistor 30. This tends to be due to the temperature sensitivity ofthe internal components of the control module 20. The calibrationprocess tends to ensure that the system automatically recalibrates tocorrect for errors caused by the temperature sensitivity of the internalcomponents, for example, if large internal temperature changes of theinternal components of the control module 20 occur.

At step s20, the internal temperature sensor 68 measures a temperaturewithin the housing 40.

At step s22, the internal temperature sensor 68 sends the measurement ofthe temperature inside the housing 40 to the processor 64.

The processor 64 may store the received internal temperature measurementat the memory 56 as performance data.

At step s24, the processor 64 determines whether or not the measuredtemperature inside the housing 40 to determine whether or not a largechange (for example, +/−10° C., or +/−20° C.) from a predefinedtemperature value or temperature range in the temperature within thehousing 40 has occurred.

Processing performed by the processor 64 at step s24 may be specified inthe control laws 78.

If at step s24, it is determined that the measured temperature insidethe housing 40 has not deviated by a large amount from the predefinedtemperature value or temperature range, the method proceeds to step s26.

However, if at step s24, it is determined that the measured temperatureinside the housing 40 has deviated by a large amount from the predefinedtemperature value or temperature range, the method proceeds to step s28.Steps s28 to s36 will be described in more detail later below after adescription of step s26.

In some embodiments, at step s24, the processor 64 performs a differentdetermination instead of or in addition to determining whether or not asufficiently large change in the temperature within the housing 40 hasoccurred. For example, the processor 64 may determine whether or not thetemperature within the housing 40 is within a second predeterminedtemperature range. In some embodiments, if the temperature within thehousing 40 is within the second predetermined temperature range, themethod proceeds to step s26, whereas if the temperature within thehousing 40 is outside the second predetermined temperature range, themethod proceeds to step s28.

At step s26, the gain of the amplifier 76 is maintained at its currentlevel.

After step s26, the process of FIG. 5 ends.

At step s28, in response to determining that the temperature inside thehousing 40 has deviated by a large amount from the predefinedtemperature value or temperature range, the processor 64 generates andsends a relay control signal to SPDT relay 69.

The processor 64 may log details of the relay control signal at thememory 56 as performance data.

At step s30, responsive to receiving the relay control signal, the stateof the SPDT relay 69 is switched. In particular, in this embodiment, thethird (common) terminal of the SPDT relay 69 is switched from beingconnected to the first terminal (and thereby to the temperaturemeasurement module 24) to being connected to the second terminal (andthereby to the reference resistor 70).

In this embodiment, the resistance of the reference resistor 70 issubstantially fixed. In other words, the resistance of the referenceresistor 70 does not vary significantly with the temperature within thehousing 40.

At step s31, the reference resistor 70 sends to an electrical signalindicative of its resistance to the processor 64 via the second EMCfilter 46, and the signal conditioning module 54. The electrical signalindicative of the resistance of the reference resistor 70 is amplifiedby the amplifier 76 of the signal conditioning module 54 prior to beingreceived by the processor 64.

At step s32, the processor 64 recalibrates the amplifier 76 of thesignal conditioning module 54.

In particular, in this embodiment, the processor 64 uses the receivedsignal that is indicative of the resistance of the reference resistor 70(that has been amplified by the amplifier 76), and the known resistanceof the reference resistor 70 to determine a recalibration for theamplifier 76. This recalibration may be such that the received amplifiedsignal that is indicative of the resistance of the reference resistor 70matches the known resistance of the reference resistor 70. The processor64 then recalibrates the amplifier 76 according to the determinedrecalibration.

The processor 64 may log details of amplifier recalibration at thememory 56 as performance data.

At step s34, after recalibration of the amplifier 76, the processor 64sends a second relay control signal to the SPDT relay 69.

The processor 64 may log details of the second relay control signal atthe memory 56 as performance data.

At step s36, responsive to receiving the second relay control signal,the state of the SPDT relay 69 is switched. In particular, in thisembodiment, the third (common) terminal of the SPDT relay 69 is switchedfrom being connected to the second terminal (and thereby to thereference resistor 70) to being connected to the second terminal (andthereby to the temperature measurement module 24). Thus, the processor64 again receives temperature measurements from the thermistor 30.

After step s36, the process of FIG. 5 ends.

Thus, an automatic calibration process of the control module 20 isprovided.

An advantage provided by the above described automatic calibrationprocess is that detrimental effects of changes in temperature within thehousing 40 on the performance of the control module 20 tend to bereduced or eliminated. For example, changes to the amplifier gain (andother temperature dependent components) within the housing unit. Thistends to allow the control module 20 to operate effectively over anincreased temperature range, for example −40° C. to +80° C.

Advantageously, the control module 20 may automatically recalibrate toaccount for undesirable internal temperature changes. The calibrationmay, for example, be performed at power-up of the control module 20,and/or when the internal temperature of the housing changes be apredetermined value (e.g. =/−20° C.).

FIG. 6 is a process flow chart showing certain steps of an icingavoidance process. The process of FIG. 6 may be performed at regularintervals, e.g. at a frequency of 20 Hz.

At step s40, the thermistor 30 measures a temperature of the conditionedair in the conditioned air channel 10.

At step s42, the thermistor 30 sends the measurement of the temperatureof the conditioned air to the processor 64.

The processor 64 may store the received conditioned air temperaturemeasurement at the memory 56 as performance data.

At step s44, the processor 64 determines whether or not the measuredconditioned air temperature is less than or equal to 0° C.

By determining that the temperature of the conditioned air is less thanor equal to 0° C., the processor 64 determines that there is a risk ofice forming within the conditioned air channel 10, and the aircraftsubsystems 6 that receive the conditioned air. In other embodiments, oneor more different criteria may be used to determine that there is a riskof icing instead of or in addition to the temperature measurement of theconditioned air being less than or equal to 0° C. For example, in someembodiments, a lower temperature threshold below 0° C. is used.

Processing performed by the processor 64 at step s44 may be specified inthe control laws 78.

If at step s44, it is determined that the measured conditioned airtemperature is greater than or equal to 0° C., the method proceeds tostep s46.

However, if at step s44, it is determined that the measured conditionedair temperature is less than or equal to 0° C., the method proceeds tostep s48. Step s48 will be described in more detail later below after adescription of step s46.

At step s46, in response to determining that the conditioned airtemperature is greater than 0° C., the processor 64 controls the valvedriver 52 such that the gain of the valve driver 52 is equal to a firstpredetermined value. The first value may be any appropriate gain valuefor the valve driver 52. The first value is a constant.

The processor 64 may log detail of its control of the valve driver 52 atstep s46 at the memory 56 as performance data.

In some embodiments, where no icing is detected in a previousperformance of the process of FIG. 6, the first predetermined value maybe a current value of the gain of the valve driver 52.

The actions performed by the processor 64 at step s46 may be specifiedin the control laws 78.

After step s46, the process of FIG. 6 ends.

At step s48, in response to determining that the conditioned airtemperature is less than or equal to 0° C., the processor 64 controlsthe valve driver 52 such that the gain of the valve driver 52 is equalto a second predetermined value. The second value is greater than thefirst value. Thus, upon first determining that the conditioned airtemperature is less than or equal to 0° C., the gain of the valve driver52 is increased. Thus, the power and/or amplitude of the valve controlsignals generated by the valve driver 52 is increased.

In this embodiment, the second value is a constant that is greater thanthe first value. However, in other embodiments, the second value is nota constant, i.e. the second value is a variable. For example, in someembodiments, the second value is a function of the measured temperature,e.g., a linear function that increases as the measured temperaturedecreases.

Increasing the amplitude of the valve control signal advantageouslytends to provide that the movement rate of the valve 22 is increased.Thus, when the processor 64 detects an icing risk (i.e. cooling airtemperatures less than or equal to 0° C.), the rate at which the valve22 moves is increased. Thus, the speed at which the valve 22 mayincrease relatively hot bleed air into the conditioned air is increased.This advantageously tends to provide that the likelihood of ice formingis reduced. Also, this advantageously tends to provide that any formedice is rapidly melted.

The actions performed by the processor 64 at step s48 may be specifiedin the control laws 78.

The processor 64 may log detail of its control of the valve driver 52 atstep s48 at the memory 56 as performance data.

After step s48, the process of FIG. 6 ends.

Thus, an icing avoidance process is provided.

The icing avoidance process tends to be performed automatically by thecontrol module 20. Also, the icing avoidance process tends to beadvantageously proactive, i.e. icing prevention steps tend to beperformed prior to ice formation.

FIG. 7 is a process flow chart showing certain steps of an oscillationavoidance process. The process of FIG. 7 may be performed at regularintervals, e.g. at a frequency of 20 Hz.

At step s50, the thermistor 30 measures a temperature of the conditionedair within the conditioned air channel 10.

At step s52, the thermistor 30 sends the measurement of the conditionedair temperature to the processor 64.

The processor 64 may store the received conditioned air temperaturemeasurement at the memory 56 as performance data.

At step s54, the processor 64 determines whether or not the conditionedair temperature measurements indicate that oscillation is occurring. Inthis embodiment, the processor 64 determines that oscillation isoccurring if, over a predetermined time period, the conditioned airtemperature measurements cross a predetermined central value more than apredetermined number of times. In other words, the processor 64determines that oscillation is occurring if, within the predeterminedtime period, the number of times that the temperature measurementschange from being above the central value to being below the centralvalue, and/or from being below the central value to being above thecentral value, is greater than a predetermined threshold value. Thispredetermined time period may, for example, be 20s. The central valuemay, for example, be 4° C. The threshold value for the crossing numbermay, for example, be 2.

Processing performed by the processor 64 at step s54 may be specified inthe control laws 78.

If at step s54, it is determined that oscillation of the conditioned airtemperature is not occurring, the method proceeds to step s56.

However, if at step s54, it is determined that oscillation of theconditioned air temperature is occurring, the method proceeds to steps58. Step s58 will be described in more detail later below after adescription of step s56.

At step s56, in response to determining that oscillation is notoccurring, the processor 64 controls the valve driver 52 such that thegain of the valve driver 52 is equal to a third predetermined value. Thethird value may be any appropriate gain value for the driver 52. In thisembodiment, the third value is a constant. In some embodiments, thethird value is equal to the first value.

In some embodiments, where no oscillation is detected in a previousperformance of the process of FIG. 7, the third predetermined value maybe a current value of the gain of the valve driver 52.

The actions performed by the processor 64 at step s56 may be specifiedin the control laws 78.

The processor 64 may log details of its control of the valve driver 52at step s56 at the memory 56 as performance data.

After step s56, the process of FIG. 7 ends.

At step s58, in response to determining that oscillation of theconditioned air temperature is occurring, the processor 64 controls thevalve driver 52 such that the gain of the valve driver 52 is equal to afourth predetermined value. The fourth value is less than the thirdvalue. Thus, upon first determining that oscillation of the conditionedair temperature is occurring, the gain of the valve driver 52 isdecreased. Thus, the amplitude of the valve control signals generated bythe valve driver 52 is decreased.

In this embodiment, the fourth value is a variable. For example, in someembodiments, the fourth value decreases as the number of oscillationsincrease above the threshold value for the crossing number. This tendsto increase the anti-oscillation action as the detected oscillationpersists. However, in other embodiments, the fourth value may be adifferent value, for example, a different variable value or a constant.

Decreasing the amplitude of the valve control signal advantageouslytends to provide that the movement rate of the valve 22 is decreased.Thus, when the processor 64 detects oscillation of the conditioned airtemperatures, the rate at which the valve 22 moves is decreased. Thus,the speed at which the valve 22 may change the relative proportions ofbleed air and cooling air in the conditioned air is decreased. Thisadvantageously tends to provide a “smoothing” effect that tends toreduce or eliminate oscillation.

The actions performed by the processor 64 at step s58 may be specifiedin the control laws 78.

The processor 64 may log detail of its control of the valve driver 52 atstep s48 at the memory 56 as performance data.

After step s58, the process of FIG. 7 ends.

Thus, an oscillation avoidance process is provided.

The oscillation avoidance process tends to be performed automatically bythe control module 20. By reducing or eliminating oscillation, themaintaining of the temperature of the conditioned air in the desiredrange tends to be facilitated.

The oscillation avoidance process tends to avoid intermittent icing fromoccurring as the oscillating temperature drops below 0° C.

The oscillation avoidance process tends to reduce or eliminate excessivewear on the valve 22 due to its constant operation.

FIG. 8 is a process flow chart showing certain steps of a process ofimproving the control laws 64 of the control module 20.

In this embodiment, the process of FIG. 8 is performed while theaircraft 2 is on the ground.

At step s60, a computer (such as a laptop computer or a tablet computer)that is separate from the aircraft 2 is connected to the control module20. The computer is connected to the input connection 71 and the outputconnection 72.

In this embodiment, the input connection 71 and the output connection 72are only accessible for connection with the computer while the aircraft2 is grounded. While the aircraft 2 is in flight, the input connection71 and the output connection 72 may be inaccessible for connection witha computer. For example, in some embodiments, while the aircraft 2 is inflight, the input connection 71 and the output connection 72 may beretracted into the housing 40, or may be covered by a cover.

At step s62, the processor 64 retrieves stored performance data from thememory 56. The performance data may include, but is not limited to,aircraft flight data, measurements taken by the thermistor 30,measurements taken by the internal temperature sensor 68, a log of thePWM valve control signals generated by the valve driver 52, a log ofrecalibrations of the control laws 78, a log of recalibrations of theamplifier 76, details of icing events, details of oscillation events,and a log of the relay control signals. The performance data may havebeen stored in the memory 56 while the aircraft 2 was in flight, e.g.during one or more aircraft sorties.

In some embodiments, step s62 is performed responsive to the computerbeing attached to the control module 20 as performed at step s60.

At step s64, the processor 64 sends the retrieved performance data tothe computer. In this embodiment, the performance data is sent from theprocessor 64 to the external computer via the transceiver 60, theisolating relay 50, the controller 42, and the output connection 72.

At step s66, the received performance data is analysed to determine anupdate (for example, an improvement or refinement) for the control laws78. For example, a new value or function for one or more of thethreshold values may be determined. Also for example, new functions orcriteria for implementation by the control laws 78 may be determined. Inthis embodiments, the update for the control laws 78 provides forimproved operation of the environmental control system 4.

In some embodiments, the analysis of the performance data comprises ananalytical/engineering design process, e.g. performed by a humanoperator using software tools.

In some embodiments, the computer performs the data analysis process onthe received performance data. For example, the computer may perform afault diagnosis process the diagnose aircraft faults. For example, thecomputer may, using the performance data, determine one or more faultswith the processor 64 or another module of the environmental controlsystem 4.

At step s68, the computer sends a configuration signal to the in-serviceprogramming module 58. The configuration signal specifies the update forthe control laws determined by the computer. In this embodiment, theconfiguration signal is sent from the computer to the in-service signalprocessing module 58 via the input connection 71, the controller 42, andthe isolating relay 50.

At step s71, the in-service programming module 58 updates the controllaws 78 of the processor 64 using the received configuration signal.

Thus, a process of improving the control laws 78 is provided.

Advantageously the control laws 78 may be updated and improved based onpast performance of the environmental control system 4. The control laws78 may be updated after each aircraft sortie until optimal operation ofthe environmental control system 4 is achieved.

The control laws 78 may be advantageously updated to improve the controlmodule's automatic recalibration of the amplifier 76 in response tointernal temperature changes.

The control laws 78 may be advantageously updated to improve the controlmodule's detection of, and/or response to, icing.

The control laws 78 may be advantageously updated to improve the controlmodule's detection of, and/or response to, signal oscillation.

Advantageously, the isolating relay 50 tends to prevent or oppose theunwanted updating on the control laws while the aircraft 2 is in flight.Also, having the input connection 71 and/or the output connection 72inaccessible while the aircraft 2 is in flight tends to prevent oroppose the unwanted updating on the control laws while the aircraft 2 isin flight.

It should be noted that certain of the process steps depicted in theflowcharts of FIGS. 4 to 8 and described above may be omitted or suchprocess steps may be performed in differing order to that presentedabove and shown in the Figures. Furthermore, although all the processsteps have, for convenience and ease of understanding, been depicted asdiscrete temporally-sequential steps, nevertheless some of the processsteps may in fact be performed simultaneously or at least overlapping tosome extent temporally.

Advantageously, the processes described above with reference to FIGS. 4to 8 tend to be able to be performed simultaneously or at leastoverlapping to some extent temporally.

In the above embodiments, the control module is implemented in anenvironmental control system of a Hawk aircraft. The control systemcontrols a mixture of bleed air and cooling air. However, in otherembodiments the control module is implemented on a different entity, forexample, a different type of aircraft. For example, in some embodimentsthe control module is to control a different type of system, i.e. otherthan an environmental control system.

In the above embodiments, an SPDT relay is used to provide switchingbetween the thermistor signal and the reference resistor signal.However, in other embodiments a different switching means is used.

In the above embodiments, the cooling air is bleed air cooled usingambient air. However, in other embodiments, the cooling air is from adifferent source and/or is cooled in a different way, e.g. cooledambient air.

In the above embodiments, the icing detection and avoidance process isperformed in combination with one or more of the following processes:the cooling temperature control process, the automatic recalibrationprocess, the oscillation detection and avoidance process, and thecontrol law updating process. However, in other embodiments, the icingdetection and avoidance process is performed, but not in combinationwith any of those other processes, i.e. the other processes may beomitted.

What is claimed is:
 1. An aircraft environmental control system forcontrolling a temperature of a fluid, the control system comprising: atemperature sensor configured to measure a temperature of the fluid andto generate a first signal, the first signal being indicative of themeasured temperature; a control signal generator configured to,dependent upon the first signal, generate a control signal forcontrolling the temperature of the fluid; and one or more processorsconfigured to, responsive to determining that the measured temperatureis less than or equal to a pre-determined threshold value, increase again of the control signal generator.
 2. The aircraft environmentalcontrol system according to claim 1, wherein the pre-determinedthreshold value is less than or equal to 0° C.
 3. The aircraftenvironmental control system according to claim 1, wherein: the fluid isa mixture comprising a second fluid and a third fluid; the environmentalcontrol system further comprises a valve for mixing the second fluid andthe third fluid; the control signal generator is configured to, usingthe first signal, generate a control signal for controlling the valve;and the one or more processors are configured to, responsive todetermining that the measured temperature is less than or equal to thepre-determined threshold value, increase a gain of the control signalgenerator, thereby increasing an amplitude and/or power of the controlsignal generated by the control signal generator.
 4. The aircraftenvironmental control system according to claim 3, wherein: the secondfluid comprises bleed air from a subsystem of the aircraft; and thethird fluid comprises refrigerated bleed air.
 5. The aircraftenvironmental control system according to claim 1, wherein the one ormore processors are further configured to: determine that the measuredtemperature is oscillating about a central value; and responsive to thedetermining that the measured second temperature is oscillating about acentral value, decrease the gain of the control signal generator.
 6. Theaircraft environmental control system according to claim 1, wherein theaircraft environmental control system further comprises: a control lawmodule storing one or more control laws, the control laws being for useby the one or more processors when generating the control signal; atransmitter configured to transmit, from the aircraft environmentalcontrol system, for use by one or more entities remote from the aircraftenvironmental control system, performance data, the performance dataincluding data selected from the group of data consisting of: one ormore measured values of the temperature, data indicative of the firstsignal, data indicative of a gain of the control signal generator, anddata indicative of the control signal; and a receiver configured toreceive, responsive to the transmitter transmitting the performancedata, from the one or more entities remote from the aircraftenvironmental control system, update information for use by the one ormore processors; the aircraft environmental control system being furtherconfigured, using the update information, to update the one or morecontrol laws.
 7. The aircraft environmental control system according toclaim 1, wherein: the control system further comprises an amplifierconfigured to amplify the first signal; the control signal generator isconfigured to generate the control signal using the amplified firstsignal; and the one or more processors are further configured to,responsive to determining that a temperature of an operationalenvironment of the amplifier is not within a predetermined temperaturerange, modify a gain of the amplifier.
 8. The aircraft environmentalcontrol system according to claim 7, wherein: the aircraft environmentalcontrol system further comprises a housing and a second temperaturesensor configured to measure the temperature of the operationalenvironment of the amplifier within the housing; and the amplifier, thesecond temperature sensor, and the one or more processors are locatedwithin the housing.
 9. The aircraft environmental control systemaccording to claim 7, wherein: the aircraft environmental control systemfurther comprises a baseline signal generation module configured togenerate a baseline signal, the baseline signal being independent of thetemperature of the operational environment of the amplifier; theamplifier is further configured to amplify the baseline signal; and theone or more processors are further configured to, responsive todetermining that the temperature of an operational environment of theamplifier is not within the predetermined temperature range, modify thegain of the amplifier using the amplified baseline signal.
 10. Theaircraft environmental control system according to claim 9, wherein: thebaseline signal generation module comprises a resistor havingsubstantially constant resistance; and the baseline signal is indicativeof a resistance of the resistor.
 11. The aircraft environmental controlsystem according to claim 9, wherein: the environmental control systemfurther comprises a relay switchable between a first mode and a secondmode; in its first mode, the relay connects the first temperature sensorto the amplifier and disconnects the baseline signal generation modulefrom the amplifier; in its second mode, the relay connects the baselinesignal generation module to the amplifier and disconnects the firsttemperature sensor from the amplifier; and the one or more processorsare further configured to, dependent on the measurements by the secondtemperature sensor, control the switching of the relay.
 12. The aircraftenvironmental control system according to claim 11, wherein the one ormore processors are further configured to, responsive to determiningthat the temperature of an operational environment of the amplifier isnot within the predetermined temperature range, control the relay toswitch from its first mode to its second mode.
 13. An aircraftcomprising an aircraft environmental control system according toclaim
 1. 14. An aircraft environmental control method for controlling atemperature of a fluid, the method comprising: measuring, by atemperature sensor, a temperature of the fluid; generating, by a controlsignal generator, a first signal, the first signal being indicative ofthe measured temperature; generating, by a control signal generator,dependent upon the first signal, a control signal for controlling thetemperature of the fluid; determining, by a processor, that the measuredtemperature is less than or equal to a predetermined threshold value;and responsive to determining that the measured temperature is less thanor equal to the pre-determined threshold value, increasing a gain of thecontrol signal generator.